A Light-Emitting Diode (LED) is a light-emitting semiconductor device based on the electroluminescence in the P—N junction of semiconductors. LEDs have the advantages including pollution-free, high brightness, low power consumption, long life cycle, low operating voltage, and easy to be small-scaled. Since the invention of gallium nitride (GaN) based LEDs in the 1990s, with the research progress, brightness has been continuously improved, and the range of application has become increasingly wider. With the continuous improvement in the efficiency of power GaN-based LEDs, it seems inevitable that GaN-based LED lamps will replace the existing lighting equipment. However, for the popularization of semiconductor lighting, many problems are still left to be solved, among which the most important one lies in the production cost and luminous efficacy.
Presently, commercially-available blue and green LEDs are based on GaN-based III-V semiconductor materials. The epitaxial growth of most GaN-based LEDs in the market is made with sapphire (Al2O3) substrates and silicon carbide (SiC) substrates. SiC substrates are expensive, resulting in high cost of LED production; and the lattice constant and coefficient of thermal expansion of sapphire are considerably different from those of GaN, causing a threading dislocation density of the GaN-based epitaxial layer as high as 108-1010 cm−2. The high dislocation density may limit further improvement of the performance of the optoelectronic devices. Therefore, in order to improve luminous efficacy and life cycle, the dislocation density between the substrate and GaN has to be reduced.
Techniques to improve the luminous efficacy of LEDs generally include: patterned substrate, transparent substrate, Distributed Bragg Reflector (DBR) structure, surface microstructure, flip-chip, chip bonding, and laser lift-off. A widely-used one is PSS (Patterned Sapphire Substrate), which includes: forming a patterned mask, normally a silicon dioxide (SiO2) or metal mask, on a sapphire substrate; etching the sapphire substrate; and removing the mask so to obtain the dot-patterned sapphire substrate (see document [1] S Kitamura, K Hiramatsu and N Sawaki. Fabrication of GaN hexagonal pyramids on dot-patterned GaN/sapphire substrates via selective metalorganic vapor phase epitaxy [J]. Jpn. J. Appl. Phys., 1995, 34: L1184-L1186; and document [2] W K Wang, D S Wuu, S H Lin, et al. Efficiency Improvement of Near-Ultraviolet InGaN LEDs Using Patterned Sapphire Substrates [J]. IEEE Photo. Technol. Lett., 2005, 17 (2): 288-290.) But due to the hardness of the sapphire substrate, it is difficult to ensure the consistency and uniformity of the patterns over the chip, which calls for high standard equipment and processing, and leads to high-priced PSS substrates and higher cost of LED production.
Chinese patent application (CN101388427) discloses a method for manufacturing a LED device, which combines epitaxy and etching, and includes etching from the bottom of the epitaxial layer of the LED, so as to form an over-hanging and protruding side of the structure and an inclined face for the epitaxial layer of the LED, thereby reducing the total reflection at the interface of the LED assembly. However, this method changes only the shape of the side of the epitaxial layer, and forms no through-hole structure, thus the improvement in light extraction is limited.